clocksource: samsung_pwm_timer: Cache clocksource register address

Instead of calculating register every time the timer should be read,
we can just do it one time at initialization and store the address in
driver data.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Mark Brown <broonie@linaro.org>
Tested-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>